Conveyors, such as escalators, travelators, moving walkways, and the like, provide a moving pathway to quickly and conveniently transport people from one location to another. More specifically, the moving pallets or steps of a conveyor move passengers along the length of the pathway between two landing platforms at predetermined rates of speed. Step chains hidden from view and disposed underneath the conveyor serve to interconnect each of the steps in a closed loop fashion. Driven by a main drive source, drive shafts and associated sprockets, the step chains move the steps along an exposed upper surface of the conveyor to transport passengers between the landing platforms. Sprockets disposed within each of the two landing platforms guide the step chains through an arc to reverse the direction of step movement and to create a cyclic return path.
Because of their continual motion, conveyors are prone to various internal failures, which may further cause injury to passengers on or near the conveyor. One of these failures pertains to misaligned or missing pallets or steps. Over time, one or more steps of a conveyor may break loose from the associated step chains causing the steps to drop or fall within the conveyor system undetected. Missing steps may also be caused by improper maintenance. Conveyors require periodic maintenance in which one or more steps may be removed, replaced, or the like. However, if a step is not properly fastened or realigned with the step chains, the step may break loose and fall. In any event, if a control system of a conveyor fails to detect a void caused by a missing step, the conveyor may continue to operate, advance the void to the upper surface of the conveyor and expose the void to passengers. Unknowing passengers may fall or step into the void and become injured. The issue of missing pallets or steps and the detection thereof is therefore well known in the art of conveyors. While there are several existing systems which provide such safety control measures for conveyors and aim to accurately detect such faults, they have their drawbacks.
Safety control systems for conveyors exist in which electromechanical switches are used to detect steps or the lack thereof. Such systems position electromechanical switches within the return path of the conveyor so as to detect a misaligned or an unsupported step. Due to gravity, an unsupported step in the return path may swing away or hang from the step chains and place the step directly in the path of the electromechanical switches. However, such electromechanical switches are unable to function properly if the step is grossly out of position or completely detached from the step chains altogether. Additionally, such electromechanical switches are significantly more prone to wear and are unreliable.
Other missing step detection systems implement photoelectric sensors which use light or the interruption thereof to monitor the steps of a conveyor. In such systems, each step of the conveyor is required to have a through-hole fully extending through the width of the step. A photoelectric beam of light is then aligned to pass directly through the hole of a step when the step is properly aligned and supported by the step chains. If a step is misaligned, the beam of light is interrupted and the control system responds to the error. One disadvantage with such a scheme is that each of the steps requires significant modifications to adapt for such photoelectric sensors, and therefore, cannot be retrofit onto conveyors that carry steps without through-holes. Furthermore, safety control systems for conveyors using photoelectric sensors are susceptible to dust, debris, or anything else that may be present or that may be present or that may collect in the through-holes over time and interrupt the light paths.
Yet another existing missing step detection system employs proximity sensors which constantly detect the presence of each passing step in the return path. Such sensors electromagnetically interact with the metal in the passing step to output a corresponding voltage or current indicating the presence or absence of the passing step. However, in cases where the steps are modified for plastic or rubber inserts, there is insufficient metal to be accurately and reliably detected by the sensors. In general, conveyor safety control systems which use proximity sensors require significant modifications to the configuration of the steps. Some proximity sensor driven safety control systems may require the top surfaces of the steps to be aligned in a linear fashion in the return path. Other systems may require the side surfaces of the steps to be linear or flat.
Among the more common proximity sensors used for detecting missing steps are capacitive and inductive sensors. Capacitive sensors continuously measure a difference in voltage, or the electric field that is formed by the sensor itself. When in close proximity to the sensor, the metal of passing steps offsets the electric field, creates a difference in voltage, and causes the sensor to output a signal corresponding to the change in the electric field. However, capacitive sensors are easily affected by sources other than the metal of a passing step, such as dust, dirt or even humidity in the air, and therefore, the electrical signals output by capacitive sensors are generally unreliable.
Many systems also implement inductive proximity sensors which are robust and more reliable than capacitive sensors. Inductive sensors continuously monitor the level of current flowing through an inductive loop within the sensor. When in close proximity to the sensor, the metal of passing steps significantly alters the current flow in the inductive loop, and causes the sensor to output a signal corresponding to the change in the inductance. As with capacitive sensors, inductive sensors output continuous signals which require an associated control system to monitor the continuous signals output by a capacitive or an inductive sensor. However, according to new standards and safety regulations for conveyor systems, safety control systems which monitor continuous signals must also incorporate costly certified sensors which gauge the integrity of the proximity sensors.
Additionally, missing step detection systems which use proximity sensors and rely on continuous signal output are dependent on parameters that are not fixed or constant, such as conveyor speed and time. For instance, using the speed of the conveyor as a frame of reference, the system sets forth an expected timeframe or window at which the next consecutive step is to be detected by the proximity sensor. From a signal processing standpoint, the proximity sensors are outputting continuous detection signals and the expected window is rather broad and vague. This makes it more difficult for the control system to accurately filter out the unwanted noise from the desired detection signal, and make an accurate decision based on the filtered signal. Furthermore, while this method may be effective when the conveyor is moving at constant speeds, it is unreliable when the conveyor is accelerating, decelerating, turned on or turned off.
Therefore, there is a need for robust safety control systems which detect misaligned or missing steps accurately, reliably and cost effectively, while in full compliance with the current safety standards and regulations. More specifically, there is a need for a missing step detection system for a conveyor which does not require costly certified sensors and is redundant, or provides its own self-check. Furthermore, there is a need for a missing step detection system that provides alternating output signals with less noise, and correlates sensor output signals to result in fixed reference values that are independent to conveyor speed and time.